In the spacecraft the second stage and the third stage are within the payload fairing. The payload fairing has a diameter of 2.9 meters.
Payload fairing is equipment used to protect the spacecraft and the third stage during boost phase when the aerodynamic forces from the atmosphere could damage the rocket.
The second stage of the Delta II is an Aero jet AJ10-118K engine. The fuel used in this engine is a combination of Aerozine 50 and nitrogen tetroxide (N2O4) oxidizer. Aerozine 50 is a 50/50 mix of hydrazine and asymmetric dimethyl- hydrazine. These fuels are hypergolic propellants (see hypergolic propellants).
When the second stage ignites, the payload fairing falls off. This second stage rocket is restartable and fires twice.
The first time when it burns, it is to insert the rocket into a low-earth parking orbit. The second stage shuts off once this is achieved. In this orbit, the rocket orbits till the right position (at which the path to mars lines up with the rocket).
Once this position is reached, the second stage engine re-fires. This refiring provides proper velocity and alignment to spacecraft. Like the first stage, the second stage rocket also moves up and down on the gimbals. To control any spinning of the second stage, a cold nitrogen gas system provides roll control during powered flight.
The inertial flight control assembly (RIFCA) performs the onboard navigation, guidance, and sequencing functions. This system is pre-programmed to do so. It senses accelerations and compares them with pre-programmed values. If there is any difference from what was expected, it gets the rocket back on the correct path. This is one of the most important systems of the rocket and hence there are two of these systems onboard to provide backup in case one malfunctions.